Expand gem security build/install instructions; add checksum
Add rubygems-openpgp video/slides by @grant-olsen, working link
Recommend against signing gems with rubygems-openpgp
Highlight gem checksum before the not-recommended openpgp
Add caveat to using gem cert signed gems
Remove most OpenPGP discussion per @grant-olson @yorickpeterse
Synch general security content with the details that follow
Merge pull request #70 from bf4/security_details

Expand gem security build/install instructions; add checksum; recommend against OpenPGP